In recent years, requirements for low fuel consumption are becoming severer in connection with the movement of global emission control of carbon dioxide which follows social requirement for energy saving and a rise in concern about environmental problems. In order to meet the above requirements, tires which are reduced in rolling resistance and have low heat build-up as tire performances have come to be demanded. While a method for optimizing a tire structure has so far been studied as a method for reducing rolling resistance of a tire, but it is carried out as the most usual method to use a material exhibiting lower heat build-up as a rubber composition.
It has so far been carried out as a method for obtaining the above rubber composition having low heat build-up to improve reinforcing fillers and rubber components.
Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black can provide rubber compositions with high abrasion resistance. When low heat build-up is intended to be achieved by using carbon black alone, it is considered that a compounding amount of carbon black is reduced or that the carbon black having a large particle diameter is used, but it is known that in both cases, abrasion resistance and grip on a wet road are not prevented from being reduced. On the other hand, it is known to use silica as a filler in order to enhance low heat build-up (for example, patent documents 1 to 4). However, particles of silica tend to be aggregated by virtue of a hydrogen bond of a silanol group which is a surface functional group of silica, and heat build-up is increased due to rubbing between silicas. Further, silica is inferior as well in a wettability with a rubber molecule, so that dispersion of silica into rubber is not good. The kneading time has to be extended in order to improve the above problem. Also, if silica is insufficiently dispersed into rubber, a rubber composition is elevated in a Mooney viscosity and inferior in processability such as extrusion. Further, since a surface of a silica particle is acidic, silica adsorbs a basic substance used as a vulcanization accelerator in vulcanizing a rubber composition to prevent the rubber composition from being sufficiently vulcanized, so that the defect that the elastic modulus is not enhanced has been involved therein.
A silane coupling agent has been developed in order to improve the above defects, but when the silane coupling agent is insufficiently reacted with silica, a reinforcing property of silica is not enhanced, so that the abrasion resistance is reduced. Further, the unreacted silane coupling agent is reacted in a kneading step of the rubber composition. The reaction of silica with the silane coupling agent can be completed by increasing the number of a kneading stage in the kneading step, but this leads to reduction in the productivity in the kneading step. Accordingly, it is tried to mix silica the surface of which is treated with a hydrophobicity-providing agent to accelerate the reaction of a silane coupling agent (patent document 1).
Further, it is disclosed in a patent document 5 to use hydrophobic precipitated silica. However, since precipitated silica subjected to complete hydrophobicity treatment is used, surface silanol groups to react with a silane coupling agent disappear, and therefore the defect that rubber is not sufficiently reinforced has been involved therein. Further, silica having an increased particle diameter is used in order to improve low heat build-up, but silica is reduced in a specific surface area by increasing a particle diameter to deteriorate a reinforcing property thereof. It is disclosed in patent document 6 to use silica having a specific form, but low heat build-up and abrasion resistance of the rubber composition are not sufficiently high.
Further, a partial ester of polybasic acid is added in order to improve dispersibility of reinforcing silica into rubber and strengthen interaction between the polymers in addition to improvement in low heat build-up to thereby provide the rubber with abrasion resistance and low heat build-up (patent document 7).
On the other hand, many modified rubbers interacting with fillers such as silica, carbon black have been developed as methods for improving rubber components. For example, proposed are rubber compositions prepared by compounding silica and/or carbon black with a modified conjugate diene base polymer obtained by reacting a conjugate diene base polymer with an imino group-containing hydrocarbyloxysilane compound (patent documents 8 to 11). However, a satisfactory modifying effect is not necessarily obtained in rubber compositions blended with silica and carbon black.